When doing image operations in Cinder we may start getting errors on
RabbitMQ and DB connections, which result in the volume service
appearing as down to the scheduler.
This is caused by file I/O operations, that in some cases block
greenthreads, preventing switching to another greenthread on I/O as they
should.
This results in many different errors, so this patch makes sure that
image operations (fetch, put, verify image, ...) that could prevent
greenthread switching are executed in native threads.
Closes-Bug: #1801958
Change-Id: I8a8fbf96875319a7e5ca167fcd2bca45f57649c8
e4de4fb33d